Boost Control

A turbocharger increases its airflow rate much faster than an internal combustion engine can adapt to the increase in airflow. If left unchecked, the turbo will almost instantaneously produce extremely high boost pressures with catastrophic results on the engine. To prevent this catastrophe, we need to regulate the boost pressure that the turbo can produce so as to prevent over boosting. This is where the wastegate and Blow-Off valve (BOV) comes in.

The Wastegate

An HKS External Wastegate

An HKS External Wastegate

A wastegate is a mechanical device that controls boost pressure by regulating the exhaust gas energy that flows around the turbine-wheel by bleeding off excess exhaust gas energy. In so doing the wastegate controls the speed at which the turbine-wheel can spins. The turbine-wheel drives the compressor-wheel, which in turn produces boost pressure. As such, the wastegate regulates the amount of pressure the compressor-wheel can create. The wastegate has a pressure actuator, which is controlled by boost pressure. In its normal position, the wastegate is held shut by a spring and the full amount of exhaust gas passes through the turbine-wheel of the turbocharger and boost pressure builds. As boost pressure builds, force is applied to the actuator and when the boost pressure exceeds the pressure of the spring, the wastegate actuator diaphragm will progressively push the wastegate open against the spring. At this point some exhaust gas will flow through the wastegate and will reconnect to the exhaust system beyond the turbocharger, effectively bypassing the turbocharger and maintaining the boost pressure at the set level.

There are two types of wastegates on the market: integral wastegates that are built into the turbo; and external or remote wastegates that are integrated into the exhaust manifold just before the turbo. The external wastegate allows for a bigger valve and a smoother flow path which provides better boost control and produces better performance. An external wastegate can also be controlled by a manual boost controller or by an electronic boost controller. The manual boost controller begins opening gradually as the boost pressure builds up. This affects the efficiency of the turbo system and increases turbo lag. An electronic boost controller, on the other hand, can be programmed to open at a preset boost pressure and can even be programmed for increased boost under certain circumstances. Needless to say, the electronic boost controller is the way to go!

The exhaust feed for an external wastegate should be integrated into the exhaust header so that it is exposed to as much of the pressure in the exhaust system as possible. This means that the pipe for the wastegate should be connected at or after the collector where all the primary exhaust pipes join together, or after the last exhaust port on a log-type header. Also, the wastegate should be located at an angle that does not restrict exhaust gas flow to the wastegate. The exhaust gas must be able to flow to the wastegate so that the wastegate can detect the correct exhaust pressure in the system. A Y-pipe that provides symmetry and easy flow paths would be ideal.

The exhaust gas flow from the wastegate into the tailpipe should also not interfere with the gas flow from the turbine. Any interference will increase back pressure! For best performance, the pipe that feeds exhaust gas from the wastegate to the tailpipe should be at least 18 inches long. A completely separate tailpipe for the wastegate exhaust gas would be even better as this provides the best wastegate response and the lowest back pressure. When going this route remember to allow for tailpipe expansion as the tailpipe from the wastegate will experience fluctuations in temperature. However, in most road cars legislation requires that your wastegate feed back into the main exhaust system ahead of the catalytic converter.

Whether you use an electronic boost controller or not, you should have an emergency boost control device in case the wastegate fails. This device can be a simple vent valve or a boost-sensitive electronic switch that cuts the fuel supply. A boost-sensitive electronic switch that cuts the fuel supply can be set to 1 or 2 psi above the wastegate setting. Should the wastegate fail, the cut-out switch will stop the fuel supply, which will cause the boost pressure to drop.

A GReddy Blow-Off Valve

A GReddy Blow-Off Valve

The Blow-Off Valve (BOV)

The Blow-Off valve (BOV), or compressor bypass valve, performs a similar function on the air inlet side of the turbocharger. The purpose of the Blow-Off valve is to relieve intake pressure and to reduce compressor surge that occurs when the opened throttle is closed suddenly as when you brake suddenly after heavy accelleration. When the throttle is closed suddenly, the airflow in the intake system is greatly reduced. This causes airflow and pressure fluctuations in the intake system that causes compressor surge. Compression surge places high loads on the compressor wheel and the thrust bearing. If compressoin surege is left unchecked, this will lead to thrust bearing failure. This is where the BOV comes in.

The Blow-Off valve (BOV) should located between the compressor outlet on the tubo and the throttle body, or between the intercooler and throttle body if and intercooler is fitted. It uses the manifold pressure signal and spring force to detect boost pressure fluctuations in the air intake. When it detects pressure fluctuations, the BOV vents the boost pressure to atmosphere to reduce compressor surge.